DE102016102683A1 - Method for controlling the printing elements of mutually offset printheads in an ink printing device - Google Patents

Abstract

The print images are stored in the printer controller as bitmaps of bitmap data arranged in rows and columns, wherein in each case a print element of a print head can be assigned to a column of the bitmap. The print elements of the print heads lying in an overlap area can be assigned the same bitmap data of the bitmap, wherein the print elements in the overlap area are controlled in such a way that a print dot assigned to a bitmap datum can be printed both by a print element of the one print head and by a print element of the other print head.

Description

For single- or multi-color printing of a printing material, for example a single sheet or a ribbon-shaped printing material made of a wide variety of materials, such as a paper web, ink printing devices can be used. The structure of such ink printing devices is known, see, for example EP 0 788 882 B1 , Inkjet printing devices, which operate eg according to the drop-on-demand (DoD) principle, have as printing unit a printhead or several printheads with ink ducts comprising nozzles whose activators under the control of a printer control excite drops of ink in the direction of the printing material, which are directed onto the substrate, to apply pressure points for a printed image there. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.

The structure of a print head, which has, for example, pressure elements with piezoelectric activators, is out US Pat. No. 7,281,778 B2 known. A pressure member includes an ink channel terminating in a nozzle plate disposed within a nozzle plate and provides an activator disposed on the ink channel. At the nozzle plate, the printing material is passed. When printing is to occur, activators for printing are driven by a printer controller, which then exposes the ink in the ink channels to pressure waves that cause ejection of ink droplets from the nozzles toward the substrate web.

In an inkjet printing system, the inkjet print heads of a color in a print bar can be arranged side by side and offset from one another to achieve a larger print width or a higher print resolution or printing speed. The printheads within a print bar then have to be mechanically aligned with each other, or the print timing of the nozzles of the print heads must be electronically adjustable so that the print dots on the substrate impinge upon the desired location dictated by the print control.

When arranging the printheads to each other on the pressure bar is to be noted that no gaps may arise in the printed image during printing. To accomplish this goal, it is known to offset the printheads so that there are nozzles from both adjacent printheads in an overlap area. Out DE 10 2010 036 957 A1 Such an arrangement of two print heads is known. Within a print head, the nozzles are arranged in rows, wherein the nozzles of adjacent lines are offset from one another in such a way that in a projection of the nozzles in the transport direction of the printing material, the nozzles are arranged directly next to one another. The nozzles of the lines of a print head are then each on diagonals.

• – Beim ersten Lösungsweg drucken im Überlappungsbereich die Druckelemente beider Druckköpfe: Diese Vorgehensweise führt dazu, dass das Druckbild im Überlappungsbereich verstärkt ist. Diese Betriebsweise wird bei flächigen Druckbildern angewandt.
• – Beim zweiten Lösungsweg feuern die Druckelemente der beiden Druckköpfe im Überlappungsbereich abwechselnd. Diese Betriebsweise führt zu einem unregelmäßigen Druckbild im Überlappungsbereich.
• – Beim dritten Lösungsweg feuern nur die Druckelemente eines der Druckköpfe im Überlappungsbereich der beiden Druckköpfe. Diese Betriebsweise wird als vorteilhaft beim Druck von Linien angesehen.

In EP 1 375 146 A1 It is described that in an ink printing device with offset and at an angle arranged printheads in an overlap region of two printheads each printing elements of both printheads are arranged. In printing operation, it should then be avoided that, for example, disturbing streaks are created by driving printing elements in the overlapping areas. To address this problem, three solutions are investigated:

• - In the first approach, the print elements of both printheads print in the overlap area: this procedure causes the print image in the overlap area to be reinforced. This mode of operation is used for areal printed images.
• - In the second approach, the print elements of the two printheads fire alternately in the overlap area. This mode of operation leads to an irregular printed image in the overlapping area.
• - In the third approach, only the print elements of one of the printheads fire in the overlap area of the two printheads. This mode of operation is considered to be advantageous in printing lines.

According to the offset printing, the print images can be displayed in a pixel grid according to known raster methods in digital printing. The pixel grid in the printer control corresponds to a bitmap of bitmap data which indicates whether the individual pixel in the pixel grid has to be assigned a "1" for the creation of a printing dot on the printing substrate or a "0" for no printing dot. In the printer controller, the print images are then stored as bitmaps, the individual bitmap data (also called print data) indicating whether or not the print element assigned to this bitmap datum fires an ink drop. The gray level or tonal value of such a screened print image is defined by the ratio of PELs having printed dots to PELs having no printed dots.

High-performance printing systems thus consist of an arrangement of several mutually offset printheads. This allows for seamless printing beyond the boundaries of the printheads, many times the width of the printheads. The print data processed in the bitmaps are assigned to the print elements in a defined coordinate system in the printer controller and printed line by line. Technically complex here is the exact adjustment of the print heads to each other, as already deviations in the order of 10 μm can lead to visible banding in evenly colored areas.

While the adjustment in the printing direction can be compensated electronically by adjusting the timing during the droplet formation, in particular the adjustment transversely to the printing direction has hitherto been achieved only by mechanical adjustment. This requires high-precision mechanical elements and a time-consuming setting process during the installation and replacement of individual printheads.

The problem to be solved by the invention is to provide a method of operating printheads within a printhead assembly having a plurality of staggered and overlapping printheads in an inkjet printing apparatus wherein the overlay effects of adjacent printheads in the overlap region can be minimized.

This problem is solved by a method according to the features of claim 1.

By means of a novel correction process in the raster process to the print data, the previous mechanical adjustment work of overlapping printheads is replaced by a locally limited correction in the bitmap data for these printheads. Then the printheads can be installed mechanically fixed to each other and with a defined overlap area to each other on the pressure bar.

The print images are stored in the printer controller as bitmaps of bitmap data arranged in rows and columns, wherein in each case a print element of a print head can be assigned to a column of the bitmap. The printing elements of the print heads lying in the overlapping area can be assigned the same bitmap data of the bitmap, whereby the printing elements in the overlapping area can be controlled in such a way that a print dot assigned to one bit datum can be printed both by a print element of the one print head and by a print element of the other print head.

Further developments of the invention will become apparent from the dependent claims.

In the simplest case, depending on the number of printing elements in the overlapping area, the associated columns of the bitmap of the printed image are doubled so that the individual printing elements in the overlapping area are imaged by their own bitmap columns. The pixels to be printed in the overlapping area can now be alternately printed by one of the two overlapping printheads.

By using a random generator, moire effects in the print image may be prevented, which might occur in the alternate mapping of bitmap data to printheads.

In addition, more complex raster algorithms are also possible, which assign the pixel to either one or the other printhead, depending on the surrounding pixel data. For example, For example, the raster algorithm may decide to print in the overlap area the print elements of the print head that prints the remainder of the print image.

Since the dot gain in print images is highly non-linear, an offset in the subpixel area of print dots printed by the print elements of the print heads in the overlap area may still result in visible transitions. To compensate for this problem, a tone curve measured at a reference printing pattern in the overlapping area can be detected. Then, the distribution of the pixels on the two printheads can be corrected by means of a correction function developed from the tone curve, which contains the subpixel spacing as an input variable. In order to achieve a slightly darker overall coloration in the overlapping area, according to the correction function, a portion of the pixels to be printed can be simultaneously printed by both printheads.

The method according to the invention thus has the following advantages:
A compensation of defects in the printed image in the overlapping printing range of two adjacent printheads is made possible by distribution of the pixels to be printed on the two printheads. In the simplest case, with overlapping nozzles of the printing elements of the printheads alternately the printing element of one or the other printhead can generate a pressure point on the substrate. Furthermore, a distribution of the pixels to be printed on the printing elements of the two print heads by means of a random number generator is possible. Additional correction algorithms can detect the distance of the print dots in the sub-pixel area and influence the pixel grid generation by compensation curves.

With reference to exemplary embodiments, which are illustrated in the schematic figures, the invention will be further explained.

Show it:

1 a schematic representation of a printhead assembly with multiple Druckriegeln in an ink printing unit;

2 a schematic representation of a pressure bar of five mutually offset printheads;

3 a partial plan view of the nozzle plates of two adjacent printheads indicating the overlapping area;

4 the representation of the method steps for the compensation of overlay effects in adjacent arranged printheads on the basis of a text;

5 a flowchart that the result of the method steps 4 represents.

Shown is after 1 from an ink printing apparatus, an ink printing unit 1 for printing on a substrate 3 , a transport unit for the substrate 3 and a printer controller 2 , Along the substrate 3 are pressure bolts 4 with printheads 5 one behind the other in the transport direction PF of the printing substrate 3 arranged arranged. For color printing, for example, one print bar can be used per color to be printed 4 be provided. The substrate 3 is done with the help of drive rollers 7 . 9 at the pressure locks 4 moved past, he lies on a roller saddle 8th with guide rollers on. At the entrance of the pressure bar unit DE, a sensor 6 be arranged, depending on the feed movement of the printing material 3 Pressure clock pulses T _D generated, the printer control 2 be fed and that of the printer control 2 for example, used to indicate the timing of ejection of ink drops at the nozzles of the individual printheads 5 determine if print data is ready for printing in the printer control 2 available. The sensor can eg as a rotary encoder or encoder roller 6 be executed by the substrate 3 is driven.

To 2 has a pressure latch 4 eg five printheads 5.1 to 5.5 on. The printheads 5 inside a pressure bar 4 are arranged offset to each other in two rows. 3 shows the overlap area 10 two adjacent printheads 5.1 and 5.2 in a plan view of the respective nozzle side 12 the printheads 5.1 and 5.2 , The ends of the printing elements DE are indicated by dots. The direction of the substrate 3 (not shown) is indicated by the arrow PF. In the overlap area 10 two adjacent printheads 5 , eg the printheads 5.1 and 5.2 , printing elements are DE1 from the printhead 5.1 and print elements DE2 from the printhead 5.2 arranged. Thus, in the overlap area 10 the printing elements DE1, DE2 both printheads 5.1 . 5.2 Pressure points on the substrate 4 generate, so that there is a risk that stripes in the printed image may arise during printing.

a

Anzahl der Düsen eines Druckkopfes

b

nomineller Überlapp in Pixeln

m

Anzahl der Druckköpfe über die Druckbreite

x = a·m

Anzahl der physikalisch zu druckenden Pixel

x' = x – (m – 1)·b

maximale Druckbreite in Pixel.

With the method according to the invention, which is explained below, the risk that eg stripes in the printed image on the substrate 4 be generated avoided. In the explanation of the invention it is assumed that an overlap area 10 each of four printing elements DE1, DE2 at the printheads 5.1 . 5.2 are provided. The overlap area 10 should be greater than the maximum sum of the mechanical tolerances in the chain of mechanical elements (precision of the production of the nozzles, accuracy of the contact points, manufacturing accuracy of the support frame). For example, for printheads 5 with a physical resolution of 1200dpi (equivalent to a nominal nozzle pitch of approximately 21 μm) and a sum tolerance of 80 μm a nominal overlap range 10 be provided by 4 pixels. The width of the bitmap to be printed is reduced according to the number of head boundaries. The print width is then:

a

Number of nozzles of a printhead

b

nominal overlap in pixels

m

Number of printheads across the print width

x = a · m

Number of pixels to be physically printed

x '= x - (m - 1) · b

maximum print width in pixels.

In 4 the process of generating bitmap data or print data for a print image DB is shown schematically; The individual steps are shown aligned with each other:
The print image DB is in 4a shown, it includes the word "test". For the print image DB, a pixel raster PR is generated according to a known raster method, from which a digital bitmap is generated from bitmap data arranged in bitmap data lines Z and bitmap data columns S, the bitmap data indicating whether the pixels on the printing substrate are in place 4 Pressure points should be generated or not. A pixel is then indicated as a blackened square if a pressure point is to be generated in its place.

In 4a For example, a pixel raster PR1 has been generated over the letter "e" and a pixel raster PR2, which is to consist of four columns S1 to S4 and six rows Z1 to Z6, has been generated for the letter "s". Corresponding to the pixel grids PR1, PR2, bitmaps are generated whose bitmap data for the character "e" and the character "s" contain a "1" if a printing dot on the printing substrate is used instead 4 should be generated, otherwise a "0". The generation of the bitmaps can be done in the printer control 2 respectively. According to the bitmaps thus the printing elements DE of the printheads 5 driven.

In 4b is basically a pressure register 11 shown, in which for the line Z2 of the pixel grid PR1, PR2, the individual bitmap data are entered for the pixel grid columns S1 to S8. Furthermore, in 4c below the pressure register 11 the overlap area 10 two adjacent printheads 5.1 . 5.2 shown are the printing elements DE11 to DE14 of the printhead 5.1 and the printing elements DE21 to DE24 of the print head 5.2 in the overlap area 10 shown. When the printing elements DE11 to DE24 correspond to the bitmap data of the print register 11 are controlled in the overlap area 10 through the respective printing elements DE of the print head 5.1 in the row Z2 in the columns S1 and S2 respectively generates a pressure point and by the respective printing elements DE of the print head 5.2 printed in the columns S2, S3, S4 each have a pressure point. The result of the pressure in 4d indicates that the character "s" has been printed over the character "e".

According to the invention, the bitmap for the character "e" in the overlapping area is proposed 10 to double per line Z For row Z2, the result is in the pressure register 11 in 4e shown. If now the pressure elements DE11 to DE14 and DE21 to DE24 via the pressure register 11 can be driven, the printing elements DE11 to DE14 or the printing elements DE21 to DE24 optionally print the printing dots corresponding to the line Z2 of the pixel grid PR.

For example, in the example of 4e the lines Z2, Z3, Z6 by the printing elements DE1 of the print head 5.1 and the line Z4 by the printing elements DE2 of the print head 5.2 printed ( 4f ). Since these printing elements DE in the overlapping area 10 Lines Z1 to Z6 of the pixel grid PB1 can be selected either by the printing elements DE1 of the print head 5.1 or by the printing elements DE2 of the print head 5.2 to be printed ( 4f ).

The print result shows the 4g , A superimposition of two different characters in the overlap area 10 is avoided.

5 shows in a flow chart step by step the procedure:
The overlap area is preparing 10 from adjacent printheads 5 detected, for example, four printing elements DE per printhead 5 may include ( 3 ). The overlap area 10 can by the constructive arrangement of the printheads 5 be predetermined.

In a further preparatory step, the print image is then rasterized and a pixel grid of raster columns and raster lines is generated, from which a bitmap is developed such that the print elements DE of a print head 5 Bitmap data to be assigned. From the bitmap results per printing element DE, whether this is a pressure point on the substrate 4 should fire or not. The bitmap is eg in the printer control 2 saved. From the bitmap is then also removed, at which points on the substrate 5 Pressure points are to be generated. The lines Z of the bitmap are eg in a print register 11 stored, wherein a register location is assigned to a printing element DE. This assignment also applies to those in the overlapping area 10 lying printing elements DE the printheads 5 ,

Step A1 and A2:

To prevent printing elements DE eg of the two print heads 5.1 and 5.2 in the overlap area 10 create overlapping pressure points, the bitmap in the lines Z is the number of overlapping in the area 10 extended printing elements DE extended; Thus, it becomes one of the number of printing elements DE in the overlapping area 10 corresponding number of bitmap data columns S inserted or the bitmap in the overlap area 10 doubled. Now you can print the DE elements of both printheads 5.1 . 5.2 in the overlap area 10 line by line the same print data are offered.

Steps A3 and A4:

After a selectable algorithm can now in the overlap area 10 the printing elements DE either of the one printhead 5.1 or the other printhead 5.2 be controlled. For example, the printing elements DE in the overlapping area 10 be driven alternately. For this, the register locations in the print register 11 be deleted alternately or in the doubled area.

Or the print data becomes the printheads 5 supplied in such a way that in the overlapping area 10 the print data only the respective printing elements DE a printhead 5 be forwarded.

By using a random generator, it is possible to prevent moire effects in the printed image which occur during the alternating assignment of the bitmap data to the print heads 5 could occur. Then, the print data becomes one of the print heads depending on the result of the random number generator 5 fed.

In addition, more complex raster algorithms are also possible which allow the pixel, depending on the surrounding pixel data, either one or the other printhead 5 assign. at Printing elements DE, which are in the overlap area 10 For example, the raster algorithm may decide, for example, that the printing elements DE of that printhead 5 Firing pressure points that prints the rest of the print image. In the example of 4 can then the character "e" exclusively by printing elements DE1 of the printhead 5.1 to be printed.

Since the dot gain on printed images is highly nonlinear, skewing of successive printing dots of the two printheads may occur 5.1 . 5.2 in the subpixel area lead to visible transitions in the printed image. With perfect overprinting by the print elements DE1 and DE2 of the printheads 5.1 and 5.2 There is a subpixel offset of "0". Starting from this "0" subpixel offset, the overlap of the printed dots may decrease up to a half pixel offset. The local coloration decreases. From an offset of half a pixel, therefore, should the generation of a pressure point of a printing element DE1 of a printhead 5.1 to a printing element DE2 of the adjacent printhead 5.2 change, for example, from the pressure element DE12 to the pressure element DE23.

• – Dann kann die Verteilung der Pixel und der Bitmapdaten auf die beiden Druckköpfe 5.1. 5.2 anhand einer aus der Tonwertkurve abgeleiteten Korrekturfunktion, die den Subpixelabstand der Druckpunkte der beiden Druckköpfe 5.1, 5.2 als Eingangsgröße enthält, gezielt korrigiert werden.
• – Aus der Tonwertkurve kann eine invers verlaufende Korrekturfunktion ermittelt werden, die den Verlust an Einfärbung kompensiert. Die Tonwertkurve kann als Berechnungsvorschrift oder als Screening-Matrize in der Drucksteuerung 2 gespeichert werden.
• – An Hand der Tonwertkurve kann eine definierte Zahl an Zeilen festgelegt werden, über die doppelt zu druckende Pixel verteilt werden.
• – An Hand der Tonwertkurve kann ein Gewichtungsfaktor bestimmt werden, der zufällig verteilt über die Rasterzeilen im Überlappungsbereich 10 eine definierte Anzahl von Pixeln von den benachbart liegenden Druckköpfen 5.1 und 5.2 drucken lässt, um den Verlust an Einfärbung zu kompensieren.
• – Wenn die Druckköpfe 5 einen Multilevel-Betrieb vorsehen, kann die von den Druckelementen DE zu feuernde Tropfengrößen entsprechend der Tonwertkurve eingestellt werden.

To eliminate this subpixel visible transition problem, a tone curve of a reference print image may overlap 10 be determined:

• - Then the distribution of pixels and bitmap data on the two printheads 5.1 , 5.2 based on the tone curve derived correction function, the subpixel spacing of the pressure points of the two printheads 5.1 . 5.2 contains as an input, to be corrected.
• - From the tone curve, an inverse correction function can be determined, which compensates for the loss of coloration. The tone curve can be used as a calculation rule or as a screening template in the print control 2 get saved.
• - Using the tone curve, a defined number of lines can be set, are distributed over the double-print pixels.
• - Based on the tone curve, a weighting factor can be determined, randomly distributed over the raster lines in the overlap area 10 a defined number of pixels from the adjacent printheads 5.1 and 5.2 print to compensate for the loss of coloration.
• - If the printheads 5 Provide a multi-level operation, the droplet sizes to be fired by the printing elements DE can be set according to the tone curve.

Overall, a slightly darker color in the overlap area 10 To achieve, according to the correction function, a proportion of the pixels to be printed from both printheads 5 be printed at the same time.

LIST OF REFERENCE NUMBERS

DB

 print image

PF

 Transport direction of the printing material web

T _D

 Print clock pulse

DE

 pressure element

PR

 pixel grid

Z

 Line of a pixel grid

S

 Column of a pixel grid

1

 Ink printing unit

2

 printer control

3

 printing material

4.1 to 4.4

 pressure lock

5

 Ink print head

6

 sensor

7

 drive roller

8th

 role saddle

9

 drive roller

10

 overlap area

11

 print register

12

 Nozzle side of a printhead

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0788882 B1 [0001]
• US 7281778 B2 [0002]
• DE 102010036957 A1 [0004]
• EP 1375146 A1 [0005]

Claims (12)

Method for controlling the printing elements of mutually offset overlapping printing heads in an ink printing device, in which adjacently arranged print heads ( 5 ) an overlap area ( 10 ), in the printing elements (DE) of both print heads ( 5 ), in which the print images (DB) in a printer control ( 2 ) are rasterized in each case in pixel raster (PR), wherein bitmaps are developed from the pixel rasters (PR) with bitmap data arranged in bitmap data columns and bitmap data rows and in each case one bitmap data column in each case at least one print element (DE) of the print heads (DE). 5 ), in which the in the overlap area ( 10 ) lying pressure elements (DE) of the printheads ( 5 ) are assigned to the same bitmap data columns of the bitmap, wherein the printing elements (DE) in the overlapping area ( 10 ) are controlled in such a way that a pressure point assigned to a bitmap data of a bitmap data column is detected both by a printing element (DE1) of the one print head ( 5.1 ) as well as a printing element (DE2) of the other printhead ( 5.2 ) can be printed. Method according to Claim 1, in which the number of printing elements (DE) per print head ( 5 ) in the overlapping area ( 10 ) is determined, in which the pressure elements (DE) in the overlap area ( 10 ) of a printhead ( 5.1 ) bitmap data columns of the bitmap are doubled so that each bitmap data of the bitmap in the overlap area ( 10 ) the printing elements (DE1, DE2) of both print heads ( 5.1 . 5.2 ), in which optionally a pressure point dependent on the bitmap datum of the bitmap of a printing element (DE1) of the one print head ( 5.1 ) or by a pressure element (DE2) of the other printhead ( 5.2 ) can be printed. Method according to Claim 1, in which the number of printing elements (DE) per print head ( 5 ) in the overlapping area ( 10 ) is determined, in which the pressure elements in the overlapping area ( 10 ) of a printhead ( 5.1 ) bitmap data columns of the bitmap are doubled so that each bitmap data of the bitmap in the overlap area ( 10 ) the printing elements (DE) of both printheads ( 5.1 . 5.2 ), in which the bitmap data of the bitmap in the overlap area ( 10 ) dependent pressure points in each case only by the pressure elements (DE1 or DE2) of one of the two print heads ( 5.1 or 5.2 ) can be printed. Method according to Claim 2, in which the printing elements (DE) of both printing heads ( 5.1 . 5.2 ) in the overlapping area ( 10 ) alternating pressure points as a function of the two print heads ( 5.1 . 5.2 ) associated bitmap data of the bitmap in the overlap area ( 10 ) To Print. Method according to Claim 2, in which the printing elements (DE) of both printing heads ( 5.1 . 5.2 ) in the overlapping area ( 10 ) controlled by a random gerator pressure points depending on the two printheads ( 5.1 . 5.2 ) associated bitmap data of the bitmap in the overlap area ( 10 ) To Print. Method according to Claim 2, in which the printing elements (DE) of both printing heads ( 5.1 . 5.2 ) in the overlapping area ( 10 ) depending on the overlap area ( 10 ) surrounding bitmap data pressure points depending on the two printheads ( 5.1 . 5.2 ) associated bitmap data of the bitmap in the overlap area ( 10 ) To Print. Method according to Claim 2, in which in the overlapping area ( 10 ) the printing elements (DE) of that printhead ( 5 ), which prints the rest of the print image. Method according to Claim 2, in which reference tone images have a tone curve in the overlapping region ( 10 ), in which a correction function is derived from the tone value curve, by means of which the printing elements (DE) in the overlapping area (FIG. 10 ) are controlled in such a way that a loss of coloring in the overlapping area ( 10 ), in which the correction function is used as a calculation rule or as a screening matrix in the printer control ( 29 is stored.  The method of claim 8, wherein the correction function inversely maps the tone curve. Method according to Claim 8, in which the distribution of the pixels on the two print heads ( 5.1 . 5.2 ) in the overlapping area ( 10 ) is determined by means of a correction function determined from the tone curve, which determines the subpixel spacing of adjacent print dots of the two print heads ( 5.1 . 5.2 ) as an input. The method of claim 8 or 9, wherein from the tone curve, a defined number of lines of the bitmap in the overlapping area ( 10 ), over which pixels to be printed twice are distributed. Method according to Claim 8 or 9, in which a weighting factor which is distributed randomly over the lines of the bitmap in the overlapping area ( 10 ) a defined Number of pixels from both adjacent printheads ( 5.1 and 5.2 ) to compensate for the loss of coloration.

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